Integrated optical heads for optical storage have heretofore been proposed. Integrated optical heads offer several advantages over conventional optical heads comprising a plurality of discrete components. Integrated heads can be made lighter and smaller than current heads because the function of several discrete optical elements is condensed onto a planar waveguide. Integrated heads are more easily manufacturable because waveguide optical components and associated electronic elements are fabricated in the same series of steps using lithography and thin film deposition.
One configuration of a typical optical head is described in the July 1986 issue of the Journal of Lightwave Technology at pp. 913 et seq. The optical head disclosed therein comprises a laser diode, a focusing or nonfocusing grating out-coupler, a return-beam beamsplitter, and data and servo detectors. Such an optical head is suitable for use in optical storage drives, such as CD-ROM, write-once, phase-change, and dye-bit drives, wherein data encoding is based upon the intensity or optical phase modulations of the return beam. In this type of optical storage drive, the optical components of the drive do not need to separate or detect the polarization state of the return beam.
By contrast, in magneto-optic storage drives, wherein the data is encoded as polarization changes, the optical components must separate and detect the polarization state of the return beam.
The most pertinent prior art of which applicants are aware is the hybrid integrated magneto-optic detection system described in Technical Digest FA-7 of a paper presented by H. Sunagawa et al., at the International Symposium on Optical Memory, held in Tokyo, Sept. 16-18, 1987. In their detection system, the polarization separation is performed by three separate parallel-arranged grating couplers. The outer two grating couplers are adjacent the edges of the aperture; i.e., the area which collects the light. These outer grating couplers couple only transverse electric (TE) polarization, whereas the grating in the center couples only transverse magnetic (TM) polarization. The two outer couplers couple only TE polarization and the central coupler couples only TM polarization because of a slight variation in pitch of only 0.0034 microns. Each coupled wave is focused to a respective detector area. A pair of detectors in each of the two outer detector areas detect the TE polarization and provide focusing and tracking information, respectively. A single detector in the central detector area detects the TM polarization. The magneto-optic signal is proportional to the difference in light intensity between the TE and TM detectors.
Although the system just described constitutes a complete magneto-optic head capable of detecting data and providing focus and track error signals, the return beam aperture is segmented; and, as a result, only about one-half of the available light is utilized. The polarization state of the entire aperture is not analyzed, thereby reducing both the lateral resolution and magnitude of the data signal. Also, unless the pitch differential between the grating couplers (0.0034 microns) is very precisely controlled, cross-talk will occur between the coupling of the TE and TM modes.
In a paper entitled "Nonlinear Polysilane Thin Films" presented in July 1988 at Tokyo University and published in the proceedings of the conference, there is disclosed a class of polysilane polymers that exhibit photo-introduced birefringence, and wherein the maximum induced birefringence is sufficient to allow fabrication of channel waveguides by photoexposure. However, a channel waveguide does not in itself provide any polarization mode separation. Although the maximum induced birefringence is stated to be approximately the same as the birefringence between TE and TM modes in unexposed films, this paper does not teach that TE/TM mode separation may be accomplished with a periodic birefringent structure, and it describes no embodiments or applications for the phenomenon.